Oxygen therapy apparatus

ABSTRACT

An oxygen therapy apparatus for supply of oxygen to patients. The device comprises an inlet port ( 17 ) for receiving high pressure oxygen from a cylinder and an outlet port ( 1 ) for supplying oxygen to a patient, typically via a single tube cannula (not shown). Three valves control the supply of oxygen to the patient: a diaphragm valve ( 42 ) senses the pressure at the outlet ( 1 ) and, in particular senses whether the patient is inhaling or exhaling. A valve ( 36 ) controls the supply of oxygen to the patient via an inlet jet ( 25 ) which is selectably closed by a piston ( 20 ) incorporating a flexible seat ( 24 ). Movement of the piston ( 20 ) is in turn controlled by a pilot valve ( 37 ) in dependence on the state of the diaphragm valve ( 42 ). The device gives a pulsed output to the patient during inhalation, but shuts off during exhalation to economise on the use of oxygen.

This invention relates to oxygen therapy apparatus for supply ofbreathable gas, for example, oxygen to patients.

Oxygen therapy is widely used in medical applications and is very widelyapplied in hospitals, with oxygen therapy capability to most hospitalbeds. However, many known oxygen therapy flow devices waste up to ⅔ ofthe oxygen delivered by the device due to the fact that the devicedelivers flow during the period when the patient is exhaling. There isfurther wastage, as only the oxygen delivered at the start of a breathgoes deep into the lungs where it is absorbed. Furthermore, thepatient's need and the available settings are often poorly matched; forexample a person needing 2.5 L/m might have to have 4 L/m, because ofthe small number of settings available. The invention thus relates inparticular to an oxygen economiser device for oxygen therapy apparatuswhich device seeks to reduce this wastage of oxygen.

Many patients are dependent on oxygen for mobility, and so have to carrycylinders lasting typically a couple of hours. An oxygen economiserdevice can be used to make the same cylinder last longer, or make a muchsmaller and lighter cylinder meet the existing time.

There are a number of oxygen economiser devices on the market, workingby one of two ways. A first is an electrically operated device whichbuilds up a reservoir of gas during exhalation and, as the patientstarts to inhale, opens a valve for a moment, giving a pulse of oxygeninto the first part of the inhaled breath. Variation in delivery isgiven by operating the unit every other breath, every third breath orevery fourth breath. This saves the oxygen, but has a limited number ofsettings, and needs batteries and associated circuitry. Additionallysuch units normally have additional controls which are undesirable forthe people using oxygen therapy.

A second device is one which incorporates what is effectively a demandvalve with a diaphragm to detect the decrease in pressure on inhalationwhich (by pilot operation, say) opens a valve for the main flow, andcloses it when exhalation ceases. This type of device has to have a twintube supplying the patient, since the resistance of the tube during flow(say 500 mm H₂O) is many times the magnitude of the signal, so is toogreat to allow the slight negative pressure signal (say 3 mm H₂O) to thediaphragm, so would close the diaphragm.

According to the invention there is provided a pneumatically operatedeconomiser device for supply of breathable gas to a patient, said devicehaving an inlet port for receiving a supply of pressurised gas and anoutlet port for delivering a supply of pressurised gas to the patient,valve means between the inlet port and the outlet port, said valve meansbeing switchable between a first position in which flow of gas from theinlet port to the outlet port is prevented, and a second position inwhich gas may flow from said inlet port to said outlet port, means formonitoring for inhalation by the patient, actuator means normallymaintaining the valve means in said first position but switching saidvalve means to said second position when the pressure at the outletport, as detected by said monitoring means, falls below a preset levelindicative of inhalation, and delay means for maintaining the valvemeans in said second position for a preset period.

Generally speaking, the breathable gas will be oxygen and, forconvenience, this will be assumed throughout the following description.However gases other than oxygen, and mixtures of oxygen with other gasesor vapours are possible.

Preferably said preset period is less than the expected period ofinhalation and advantageously it is considerably shorter than theinhalation period. A typical preset period is about 0.5 seconds.

The monitoring means is preferably operable to continuously monitor theoutlet pressure during exhalation, and to sample it at regular spacedintervals sufficiently small not to cause delay in the switching of thevalve means to the open position during inhalation.

At the end of the preset delay period, the valve means reverts to theclosed condition but if, at this point, the monitoring means is stillindicating that inhalation is taking place, the actuator means willimmediately re-open the valve to allow the supply of oxygen torecommence. This opening and closing of the valve means will continueuntil such time as, at the end of a preset delay period, the monitoringmeans indicates that exhalation has commenced. During exhalation, thevalve means remains in said first position—i.e. closed.

Thus, during inhalation, the patient receives a pulsed flow of oxygenhaving a period equal to the aforementioned preset period. Duringexhalation no oxygen is supplied, thus resulting in a significant savingof oxygen over conventional oxygen therapy devices which do not useoxygen economiser techniques.

This invention allows the use of single tube cannulas and single tube isface masks, as used in conventional (non oxygen economiser) oxygentherapy devices. Known oxygen economiser devices make use of two tubesleading to the patient, one to supply the oxygen, and one to monitor thestatus of the breathing cycle. The device of the invention is thus ableto utilise existing (single tube) cannulas which are more comfortablefor the patient, and do not result in supply being tied to a particularmanufacturer.

In the present invention the outlet port is two way, and therefore bothtransmits the outgoing oxygen to the patient, and receives a pressuresignal resulting from the exhaled breath from the patient. The monitoris operable to monitor the pressure at the outlet port and thusinevitably monitors both the pressure of oxygen during inhalation andthe pressure of exhalation. Four conditions at the outlet port can beidentified:

1) The valve means is closed and exhalation is taking place. In thiscase the monitored pressure is likely to be relatively high, thuscausing the actuator means to maintain the valve means in the closedcondition.

2) The valve means is closed and inhalation is taking place. In thiscase the monitored pressure is relatively low, and probably slightlynegative with respect to atmospheric, and this causes the actuator meansto open the valve means for the preset period.

3) The valve means is open and exhalation is taking place. Thiscondition is possible only when the delay means is maintaining the valvemeans open for the preset period and, during the period, the patient hasswitched from inhalation to exhalation. Once the valve means closes atthe end of the preset period, the condition will revert to (1) above andwill remain so until inhalation recommences.

4) The valve means is open and inhalation is taking place. In this casethe monitored pressure is relatively high due to the pressure of oxygenbeing supplied to the patient but, despite this, the valve remains inthe open condition for the remainder of the preset period.

It will be seen from the above that it is important with the “singletube” arrangement that the supply of oxygen is shut off at regularintervals during inhalation in order to allow the monitoring means tocheck for continued inhalation. During supply of oxygen to the patientthe small negative pressure of inhalation is swamped by the pressure ofthe oxygen itself and it is not until the supply is halted that themonitor means is able to properly detect whether the patient is inhalingor exhaling.

Preferably said valve means comprises a movable member movable between afirst position in which the valve means is closed and a second positionin which the valve means is open and wherein said actuator means isoperable upon sensing inhalation, to move said movable member from saidfirst position to said second position and wherein said delay means isoperable to cause said movable member to move back from said secondposition to said first position over a period equal to said pre-setperiod.

The movable member can take a number of forms, for example, a diaphragmor a piston. In a preferred embodiment of the invention, the movablemember takes the form of a piston which is movable within a cylinder andis subject to balancing forces as between a biassing means, for examplea spring on the one hand and pressure from said actuator means on theother. Preferably the pressure from the actuator means is gas pressureapplied to the opposite side of said piston to the biassing means. Thussaid actuator means comprises means for altering the gas pressureapplied to said piston which results in movement of said piston, againstthe force of said biassing means, from said first position to saidsecond position, or vice versa. For example, in one embodiment, themeans for altering the gas pressure is operable to increase the gaspressure on said opposite side of the piston, thus moving the pistonagainst the force of said biassing means, this movement being, in thiscase from the first (closed) position to the second (open) position. Inanother embodiment, the means for altering the gas pressure is operableto reduce the gas pressure on said opposite side of the piston, thusallowing the piston to move by the force of said biassing means, thismovement being in this case from the first (closed) position to thesecond (open) position.

Preferably the means to alter the gas pressure comprises a further valvemeans operable to supply or withdraw gas pressure to or from saidopposite side of said piston. Said valve means may be of any suitabletype, for example a diaphragm valve or a piston-operated valve.

Thus, in one embodiment, means are provided for pressurising saidopposite side of said piston, against the force of said biassing means,with sufficient pressure into said first position to normally maintainthe first-mentioned valve means in the closed position, thereby cuttingoff the oxygen supply to the patient. A reduction in pressure at theoutlet, indicative of inhalation, causes said further valve means toopen which vents the opposite side of the piston, thus reducing thepressure and allowing the first-mentioned valve means to switch to theopen position. This in turn results in an increased pressure at theoutlet port, which increased pressure causes said further valve means toclose again, thus cutting off the vent. Meanwhile, said pressurisingmeans continues to supply gas to the opposite side of the piston sothat, after a period governed by the speed at which the pressurisingmeans is able to introduce gas to said opposite side of the piston, thefirst-mentioned valve means closes again, thus cutting off the supply tothe patient, and the cycle repeats. This is explained in more detailhereinafter. Preferably the pressurising means incorporates a flowrestrictor so as to increase this period. In practice, a period of about0.5 seconds is typical.

In an alternative embodiment, the actuator means comprises two valveswhich act in tandem: A first valve acts to sense the pressure at theoutlet port. A second valve is operable to switch the application of gaspressure to said opposite side of said piston. In this case, the firstvalve, which may for example be a diaphragm valve, actuates the secondvalve, which may for example be a piston-operated valve, to supply gaspressure to the opposite side of the piston of said first mentionedvalve means, or not, as the case may be. The exact interrelationship ofthe first and second valves, and the first-mentioned valve means, isexplained in more detail hereinafter.

A reservoir can be incorporated in the supply of gas to the firstmentioned valve means in order to provide a pulse of increased pressureat the beginning of the inhalation period. The reservoir is preferablysupplied via a flow restrictor, so that the reservoir and flowrestrictor operate in tandem to enable the characteristics of theincreased pressure at the beginning of inhalation to be tailored torequirements.

In order that the invention may be better understood, an embodimentthereof will now be described by way of example only and with referenceto the accompanying drawings in which:

FIGS. 1 to 4 are sectional diagrams each illustrating one of fourdifferent embodiments of the invention;

FIG. 5 is a graph of flow against time for the embodiments of FIGS. 1and 2; and

FIG. 6 is a graph similar to FIG. 5, but in respect of the embodimentsof FIGS. 3 and 4.

Referring to FIG. 1, the oxygen economiser device comprises a block 40in which are formed a number of passages which interconnect the threebasic components of the device which are a diaphragm valve 42, apressure actuated valve 36 and a pilot valve 37.

Oxygen is input to the device from an oxygen supply, for example anoxygen cylinder (not shown) at an inlet port 17 and is split alongpassages 16 and 27 to the pressure actuated valve 36 and pilot valve 37respectively. The supply to the pressure actuated valve 36 is taken viaa variable flow restrictor 26. A further passage 14 takes the inputsupply via a flow restrictor 15 to a chamber 33 and thence to the inputof the diaphragm valve 42. The flow restrictor 15 may be made variable,for example, in the form of multiple selectable orifices.

The low pressure output to the patient is taken via a two-way outletport 1 to a single tube cannula or single tube face mask (not shown).

The pilot valve 37 comprises a T-shaped inlet piston 7 which moves in astepped bore 12 and is biased towards the top of the bore by a coilspring 11. The piston is sealed by “O” rings 8 and 9 with respect to thewider and narrower sections respectively of the bore 12. High pressureair enters the bottom of the bore 12, beneath the piston 7, via an inletjet 13 and low pressure air exits from the bore 12 via a passage 18. Aseat 10 of resilient material, such as nylon, is formed at the bottomend of piston 7 and acts to close off the jet 13, preventing orrestricting flow therethrough, when the piston 7 is at or near itslowermost position.

The space above the piston 7 defines the aforementioned chamber 33 whichhas an output connection to an inlet jet 6 to the diaphragm valve 42.The diaphragm valve comprises a diaphragm 3 which passes across andnormal seals the outlet to the jet 6. The diaphragm 3 extends across adiaphragm chamber 35 and divides the chamber into an upper pressurisedpart which is connected by passage 2 to port 1, and a lowerunpressurised part which is vented to atmosphere at port 34. The force,and therefore the pressure needed to lift the diaphragm 3 from theoutlet of jet 6, may be varied by means of a spring 5 and threaded handwheel 4.

The pressure activated valve 36 comprises a T-shaped outlet piston 20which moves in a stepped bore 19 and is biased towards the bottom of thebore by a coil spring 21. The piston is sealed by “O” rings 22 and 23with respect to the wider and narrower sections respectively of the bore19. High pressure air enters the bottom of the bore 19, beneath thepiston 20, via an inlet jet 25 and low pressure air exits from below thepiston 20 via a passage 30 to the port 1. A seat 24 of resilientmaterial, such as nylon, is formed at the bottom end of the piston 20and acts to close off the jet 25, preventing or restricting flowtherethrough when the piston 20 is at or near its lowermost position.

The head of the piston 20 divides the bore 19 into an upper chamber 32and a middle chamber 41. The upper chamber 33 is maintained atatmospheric pressure by means of a free-flow vent 31 and the middlechamber 41 is vented via a passage 28 and flow restrictor 29 to thepassage 30.

The operation of the oxygen economiser device will now be explained indetail.

During inhalation, the patient draws oxygen from the outlet port 1 andthrough the single tube cannula or single tube face mask. Duringexhalation, the flow that the patient produces passes back down thesingle tube to the outlet port 1 and proceeds up the inlet passage 2 tothe upper part of the diaphragm chamber 35. The output flow from thepatient pressurises the diaphragm 3 and forces the diaphragm 3 onto thepilot jet 6. This effectively closes off the hole in the pilot jet 6thus prohibiting flow from it. Due to the prohibition of flow throughthe pilot jet 6 the pressure within chamber 33 will increase thusforcing the inlet piston 7 down against the force of the coil spring 11.The chamber 33 is pressurised by a flow, restricted by flow restrictor15, from the inlet port 17. The piston 7 is thus forced onto the inletjet 13 and flow through this is prevented by the nylon seat 10. Thismeans that there is zero flow through the passage 18 from the pilotvalve 37 to the middle chamber 41 of the pressure actuated valve 36.

During the period that the patient is exhaling, the outlet piston 20 inthe pressure actuated valve 36 is maintained in the lower position bythe force of coil spring 21, thus sealing off the outlet jet 25 by meansof the nylon seat 24. While the piston 20 is in this position iteffectively seals off any flow to the outlet passage 30 and the middlechamber 41 is vented through the vent passage 28.

When the patient inhales through the single tube cannula or single tubeface mask then a small negative pressure is formed within the diaphragmchamber 35, causing the diaphragm 3 to lift. The pressure required tolift the diaphragm 3 may be varied by turning the hand wheel 4. If thehand wheel 4 turned in a direction to thread it into the body, then thenegative pressure required to lift the diaphragm 3 is increased. Due tothe diaphragm 3 lifting, flow is now permitted to pass through the pilotjet 6 which in turn cause the pressure within the chamber 33 of thepilot valve 37 to fall. Any flow that passes through the pilot jet 6 isvented to atmosphere through the vent port 34. This fall in pressure inthe chamber 33 allows the inlet piston 7 to rise, then breaking the sealbetween the inlet jet 13 and the nylon seat 10. This in turn allows theinlet flow from the inlet port 17 to flow through the passage 18 intothe middle chamber 41 of the pressure actuated valve 36, thus causing apressure increase within the chamber 41. This causes the outlet piston20 to rise from the inlet jet 25 at the bottom of the piston 20 in thepressure actuated valve 36 thus permitting flow of the oxygen from theinlet port 17 through the passage 27 and flow restrictor 26 to the inletjet 25. The flow restrictor allows an adjustable flow to the patient offrom 0 to 15 liters/min through said outlet jet 25 and, via passage 30and outlet port 1, to the patient through the single tube cannula orsingle tube face mask.

The rise in pressure in the passage 30 is sensed in the upper part ofthe diaphragm chamber 35 and causes the diaphragm once more to close offthe jet 6. This in turn raises the pressure in chamber 33, causingpiston 7 to lower and thus cut off the supply through jet 13 to passage18 into the middle chamber 41 of the valve 36. The incoming supply ofoxygen to the middle chamber 41 is thus terminated. Meanwhile, oxygenwithin the chamber 41 is continually leaking away, at a controlled rate,via the vent passage 28 and flow restrictor 29 to the passage 30. Oncethe incoming supply of oxygen is terminated, due to closure of the pilotvalve 7 in the manner just described, this leakage starts to cause acontinuous fall in the pressure within chamber 41 and consequentlymovement of the piston 20 in the downwards direction by the force of thecoil spring 21. Eventually the flow through the inlet jet 25 is closedoff, and flow from inlet port 17 to outlet port 1 ceases. The valve 36thus effectively incorporates a time delay function which in practice isset to approximately 0.5 seconds, thus allowing oxygen to flow to thepatient for this period. Upon closure of the pressure actuated valve 36the pressure in passage 2 is sensed by the pilot valve 37 in the mannerdescribed above.

What happens next depends upon whether the patient is still inhaling, orhas started to exhale. If the patient is still inhaling then thepressure in the upper part of chamber 35 will fall and the diaphragm 3will be lifted again substantially instantaneously, allowing the flow toresume to the patient. If the patient is by now exhaling, then thediaphragm valve 42 will not be re-operated, causing the flow to thepatient to cease. This means that during the period that the patient isinhaling there is a series of pulses of oxygen flow to the patient. Thisis clearly illustrated in FIG. 5. This action closely matches the effectof conventional (double tube) oxygen therapy whilst remaining compatiblewith current single tube cannulas and single tube face masks.

Referring now to FIG. 2 there is shown a second embodiment of theinvention. The same reference numerals have been used again whereappropriate.

The oxygen economiser device illustrated in FIG. 2 comprises a diaphragmvalve 42 and a pilot valve 37, both constructed substantially asdescribed above with reference to FIG. 1, so further explanation isomitted. The variable flow restrictor is placed in the input passageleading from the inlet orifice 17 to the jet 13. The passage 18 takingthe output of the pilot valve is passed directly to the two-way outletport 1 and the passage 2, communicating with the upper part of thediaphragm chamber 35, leads off it.

The operation will be apparent from the description given above of FIG.1, the main difference being that the 0.5 second delay is achieved bythe flow restrictor 15 controlling the time taken for the chamber 33 tobe re-pressurised.

Briefly, on exhalation, the positive pressure sensed in the upper partof chamber 35 ensures that the nozzle 6 remains shut off, and thepressure in chamber 33 is relatively high, being supplied from the inletport 17 via the passage 14 and flow restrictor 15. As a result, thepiston 7 is forced down, and the resilient seat 10 closes off the jet13, thus shutting off the supply to the patient.

Upon inhalation, the lowered pressure in the upper part of chamber 35causes the jet 6 to be uncovered, and the chamber 33 is vented toatmosphere, thus lowering the pressure in the chamber and causing thepiston 7 to be raised, thus opening the pilot valve and allowing oxygento pass from the inlet port 17 to the outlet port 1 at a ratecontrolled, as described above, by the variable flow restrictor 26.

The resultant rise in pressure in the passage 18 is sensed in the upperpart of the chamber 35 and the diaphragm valve 42 thus closes. Once thisoccurs, the pressure in chamber 33 starts to rise, supplied from theinlet port 17 via the passage 14 and flow restrictor 15. As the pressurein the chamber 33 rises, the piston 7 lowers and eventually theresilient seat 10 closes off the jet 13, thus cutting off the flowapproximately 0.5 seconds after it started.

What happens next depends upon whether the patient is still inhaling, orhas started to exhale. If the patent is still inhaling, then thediaphragm valve 42 opens again, thus re-starting the flow through thepilot valve 37 for a further 0.5 seconds. If the patient is by nowexhaling, then the diaphragm valve remains shut until the nextinhalation. The flow graph shown in FIG. 5 applies.

The embodiments of FIGS. 1 and 2 achieve essentially the same object butthe FIG. 2 embodiment uses fewer components. The FIG. 1 embodiment hasthe advantage that the valve 37 only has to supply a pilot flow to thediaphragm valve 42 so its piston can be made smaller and the volume ofchamber 33 made smaller, thus reducing the time the diaphragm has to beopen in order to switch.

As mentioned above, there can be benefits in supplying a higher rate offlow at the beginning of the inhalation cycle, this being illustratedgraphically in FIG. 6. The embodiments of FIGS. 3 and 4, approximatelyequivalent to FIGS. 1 and 2 respectively, are intended to achieve this.Once again, like reference numerals have been used where appropriate,and the following description highlights just the differences.

In the FIG. 3 embodiment, the device of FIG. 1 is modified by theaddition of a reservoir 38 in the input to the pressure actuated valve36. The connection between the reservoir 38 and the inlet port 17 is notvia any valves so the reservoir is free to fill up during the whole ofthe inhalation/exhalation cycle. However, because the exhalation periodis much longer than the individual (0.5 second) inhalation periods, thereservoir is able to fill up more during exhalation than during thebrief inhalation periods. When the valve 36 opens, oxygen is supplied tojet 25 both from the inlet orifice 17 and from the reservoir 38.However, the supply is taken preferentially from the reservoir while thereservoir pressure is the higher of the two and therefore by carefullybalancing the capacity of the reservoir 38 with the resistance to flowgenerated by the variable flow restrictor 26, a delivered flow similarto that shown in FIG. 6 can be achieved.

In the embodiment of FIG. 4, the reservoir 38 is connected between thevariable flow restrictor 26 and the jet 13. Its operation, inconjunction with the flow restrictor 26 will be apparent without furtherexplanation.

In alternative embodiments to FIGS. 3 and 4, the output of the reservoir38 is taken to a second jet (not shown), separate from the jet 25 ofFIG. 3 or the jet 13 of FIG. 4 but which is positioned so as to besealed by the respective seat 24 or 10. The reservoir supply thusbecomes separate from the supply from the inlet port 17 and this wouldenable a combination of characteristics to achieve a desired breathingtrace.

The jet assemblies 10/13 and 24/25 do not have to be of the form shown;for example each assembly may take the form of a hole with a tapered pinseated in it to selectably seal the hole.

What is claimed is:
 1. A pneumatically operated economiser device forsupply of breathable gas to a patient, said device having an inlet portfor receiving a supply of pressurised gas and an outlet port fordelivering a supply of pressurised gas to the patient, valve meansconnecting the inlet port to the outlet port, means for monitoringwhether the patient is inhaling or exhaling, said monitoring meanshaving an inlet passage connected to said outlet port whereby thepressure of gas at said outlet port may be monitored, actuator means forcausing the valve means to open so as to supply breathable gas to thepatient in the event that the patient is inhaling, and delay means forcausing the valve means to close after a preset period, thereby shuttingoff the supply of gas to the patient for a monitoring period so as toallow the monitoring means to check for continued inhalation and whereinin the event that the patient is still inhaling, the actuator meanscauses the valve means to open to re-establish the supply of gas to thepatient.
 2. A device as claimed in claim 1 in the event that the patientis exhaling during said monitoring period said actuator means causes thevalve means to remain in the closed condition.
 3. A device as claimed inclaim 1 wherein said valve means is switchable between a first positionin which flow of gas from the inlet port to the outlet port is shut off,and a second position in which gas may flow from said inlet port to saidoutlet port, and wherein said actuator means switches the valve meansfrom said first position to said second position when the monitoringmeans detects inhalation by the patient.
 4. A device as claimed in claim3 wherein said monitoring means is such that, when the pressure at theoutlet port during said monitoring period falls below a preset level,this is taken as indicative of inhalation by the patient.
 5. A device asclaimed in claim 4 wherein the actuator means is operable to switch thevalve means to said first position at the end of the monitoring period;wherein, after the preset period, the monitoring means monitors thepressure at the outlet port, and wherein the actuator means is operableto switch the valve means back to said second position should thepressure at the outlet port, as detected by said monitoring means, bebelow said preset level indicative of inhalation, and wherein said delaymeans once again maintains the valve means in said second position forsaid preset period.
 6. A device as claimed in claim 5 wherein the presetperiod is sufficiently small as to enable multiple switching cycles ofsaid valve means between said first and second positions duringinhalation, resulting in a pulsed flow of breathable gas to the patient.7. A device as claimed in claim 5 wherein the actuator means is operableto maintain the valve means in said first position should the pressureat the outlet port, as detected by said monitoring means during saidmonitoring period, be above said preset level, indicative of exhalation.8. A device as claimed in any claim 3 wherein said valve means comprisesa movable member movable between a first position in which the valvemeans is closed and a second position in which the valve means is openand wherein said actuator means is operable, upon sensing inhalation, tomove said movable member from said first position to said secondposition and wherein said delay means is operable to cause said movablemember to move back from said second position to said first positionover a period equal to said pre-set period.
 9. A device as claimed inclaim 8 further comprising means for supplying gas pressure to saidmovable member, and wherein said actuator means comprises means foraltering the gas pressure applied to said movable member which resultsin movement of said movable member from said first position towards saidsecond position or vice versa.
 10. A device as claimed in claim 9wherein the means to alter the gas pressure comprises a further valvemeans operable, upon a reduction in pressure at the outlet port to belowsaid preset level, to cause the gas pressure applied to said movablemember to be vented, thus resulting in movement of said movable memberfrom said closed position towards said open position.
 11. A device asclaimed in claim 9 wherein said means for supplying gas pressure to saidmovable member includes a flow restrictor whereby the rate of supply ofgas can be reduced to provide for said preset period.
 12. A device asclaimed in claim 9 wherein the means to alter the gas pressure comprisestwo valves which act in tandem, namely a first valve which acts to sensethe pressure at the outlet port and a second valve which is operable tocontrol the application of gas pressure to the movable member.
 13. Adevice as claimed in claim 12 wherein vent means are provided to ventthe gas pressure applied to said movable member, said vent meansincorporating a flow restrictor to limit the rate of venting to providefor said preset period.
 14. A device as claimed in claim 1 furthercomprising a reservoir connected to the inlet port.
 15. A device asclaimed in claim 14 in which a flow restrictor is placed in theconnection to the reservoir.
 16. A pneumatically operated economiserdevice for supply of breathable gas to a patient, said device comprisingan inlet port for receiving a supply of pressurised gas and an outletport for delivering pressurised gas to the patient; a selectivelycontrollable flow valve system connecting the inlet port to the outletport operable to provide breathable gas from said inlet port to saidoutlet port in pulses such that a monitoring interval is defined betweenpulses when said valve system operates to provide breathable gas fromsaid inlet port to said outlet port; a monitoring system in directcommunication with said outlet port for sensing inhalation andexhalation of a patient when a pulse of gas is not being provided tosaid outlet port; and said monitoring system operatively associated tocontrol said flow valve system when a pulse of gas is not being providedto said outlet port such that, when inhalation is sensed at said outputport, said flow valve system is permitted to operate and, whenexhalation is sensed at said output port, said flow valve system is shutoff.
 17. A device as claimed in claim 16 wherein said selectivelycontrollable flow valve system includes a pneumatically controlledbreathable gas flow valve connecting said inlet port to said outlet portoperable and a pneumatic control for said flow valve having at least oneflow restrictor for defining a pulse flow of gas through said flow valvewhen said selectively controllable flow valve system is operated toprovide breathable gas from said inlet port to said outlet port.
 18. Adevice as claimed in claim 17 wherein said monitoring system includes apneumatically controlled diaphragm valve having a control passage indirect communication with said outlet port, said diaphragm valveoperatively connected to said flow valve system to permit and shut offoperation thereof.
 19. A device as claimed in claim 18 wherein saidpneumatic control for said flow valve includes a pilot valve.
 20. Adevice as claimed in claim 18 further comprising a reservoir ofbreathable gas connected to said inlet port.